Add an iso_image target (iso-image as a target name doesn't play well with
bsd.subdirs.mk) in distrib/makefile, which builds an iso image for $MACHINE
with binary sets, stored in ${RELEASEDIR}/iso. The image is bootable for:
alpha, amd64, cats, i386, pmax, sgimips, sparc, sparc64, sun3, vax.
mac68k/macppc no there yet because of missing feature in makefs.
call iso_image in distrib/ for iso-image in the top Makefile.